It is known that several derivatives of 1,3-indanedione, particularly those disclosed in the Applicants' French Pat. No. 1,269,638, have the property of lowering the amount of prothrombin in the blood and as a result can be used as rodenticides, because they bring about in rodents a high mortality rate due to internal haemorrhage. Such anticoagulant substances are usually mixed with a carrier which can be eaten by the rodents, for example a cereal, to form a bait. A disadvantage of such a bait is that on prolonged storage it becomes heterogeneous with the result that the rodenticidal substance accumulates towards the bottom of the bait, so that the uniform rodenticidal activity of the bait is lost.